1. Field of the Invention
This invention relates to a speed control system for fans, and more particularly to an integrated circuit fan controller to control the operation and speed of direct current (DC) fans.
2. Description of the Related Art
Many modem electronic systems require forced air cooling to regulate their internal operating temperatures. Most commonly, fans, such as brushless DC fans, are utilized to control or adjust the internal operating temperature of electronic systems. In a sophisticated system, such as computers and computer related equipment, the internal operating temperature is a dynamic variable, constantly changing depending on the operation that the system is performing. Accordingly it is desirable to actively control and regulate the speed of the fan. The benefits for actively controlling the speed of the fan include longer fan life and lower acoustic noise.
A variety of solutions have been provided for controlling the speed of a fan, among them include systems implementing xe2x80x9cbit-bangingxe2x80x9d and digital-to-analog converters (DAC) to control I/O lines digitally interfaced with a control circuit of a fan. xe2x80x9cBit-bangingxe2x80x9d is a commonly used term of art referring to the manipulation of individual I/O bits under the control of software to produce the desired pattern of logic states. The software, by manipulating a desired combination of the I/O lines, can vary the voltage supply to the fan. However, software manipulation of I/O lines is time consuming and expensive. Moreover, use of software is wasteful of system resources and prone to committing errors. DAC circuits require a mixed-signal or combination of analog and digital circuitry. Mixed-signal circuitry is generally expensive to design, produce, test and calibrate.
Accordingly, a fan control system is desired which improves and simplifies the use of digitally interfaced lines for controlling or adjusting the speed of a fan.
The present invention provides a fan speed control system configured to control the operation and speed of a fan (e.g., a brushless DC fan). The fan speed control system comprises a power control block for supplying an output voltage to the fan. The power control block can be a closed-loop regulator wherein a selected portion of the output voltage, referred to as a feedback voltage, is supplied back to the power control block. The power control block compares the feedback voltage to an internal reference voltage so as to maintain the output voltage at a desired level. A voltage VDD, indicative of a logic one, is supplied to the power control block via a shutdown (SHDN) input terminal, thus enabling power control block to supply the output voltage.
The fan speed control system additionally comprises a resistor divider circuit interfaced with the power control block. The resistor divider circuit includes a plurality of I/O lines, each having a resistor connected thereto. The resistor divider circuit can selectively divide up the output voltage, thus controlling the operation and speed of the fan by varying the amount of voltage that the fan receives.
An integrated circuit fan controller is provided for drivingxe2x80x94i.e., asserting and deassertingxe2x80x94the I/O lines of the resistor divider circuit. The integrated circuit fan controller comprises a logic state machine coupled to the I/O lines via drivers, which include buffers and transistors. A timing reference or an oscillator controls the proper sequencing and timing relationship of the logic state machine. A data register coupled to a host, via a data bus, receives values indicative of control or speed operands for the fan. The logic state machine translates these values and provides signals indicative of logic one or zero to selected drivers. Accordingly, the logic state machine causes the resistor divider circuit to selectively divide up the output voltage by driving (i.e., asserting or de-asserting) none, all, or a combination of the I/O lines.
The fan speed control system additionally includes a shutdown line coupled to the SHDN input terminal. The logic state machine is coupled to the shutdown line via a driver. The logic state machine, by providing a signal indicative of logic one to the driver, causes the driver to short the shutdown line to ground to prevent voltage VDD from being supplied to the power control block. As a result, the power control block is disabled and the output voltage is prevented from being supplied to the fan.
In accordance with another aspect of the present invention, a method is provided for turning on the fan and adjusting the speed to a selected level. The method comprises the following steps: causing the power control block not to supply an output voltage to the fan; controlling the resistor divider circuit to divide the output voltage to be supplied to the fan; causing the power control block to supply the output voltage to the fan; and controlling the resistor divider circuit to divide up to a selected portion of the output voltage.
In accordance with another aspect of the present invention, a method is provided for turning off the fan. The method comprises causing the power control block not to supply an output voltage to the fan, and controlling the resistor divider circuit not to divide any of the output voltage.